The present invention relates to a process for coding track circuits and particularly for allowing the transmission of information to a vehicle travelling on a railway track divided into a series of sections each equipped with a track circuit, in which each of the carrier frequencies for energizing the track circuits is frequency modulated by a number of modulation frequencies each representing a piece of information to be transmitted. The invention also relates to a transmitting device for implementing this process.
It is known that the safety and regularity of trains travelling over railway tracks are ensured by signals spaced along the tracks which serve to transmit information for receipt by the drivers of the trains.
At the present time, these signals are generally controlled automatically by safety devices, known under the name of track circuits, which ensure safety by the division of the track into sections, the track being divided into a series of sections each equipped with a track circuit. Of course, the division into sections must take into account the length of the trains, the speed of the fastest trains and the characteristics of the track, so as to guarantee the safety of spacing between successive vehicles.
In its most current version, the track circuit includes a transmitting member and a receiving member, each connected to the rails forming the track, so that the presence of a shunting axle located between the transmitting point and the receiving point on the track causes the de-energization of a relay associated with the receiver. When several track circuits succeed each other along the track, it is obviously indispensable that the signals transmitted in one of the track circuits cannot inopportunely energize the receiver of an adjacent track circuit. This requires the propagation of signals in each track circuit to be limited. This limitation may be achieved by using for example different frequencies or polarities for two consecutive track circuits or by limiting the propagation by means of insulating joints or by bandpass circuits equivalent to short circuits between the rails for the frequencies whose propagation it is desired to limit.
There now exists high speed trains where the driver only has very little time to notice a given signal, thus increasing the risks of accidents. Furthermore, with very high-speed trains, the use of visual signals for transmitting information to the driver is absolutely out of the question.
Furthermore, the appearance of locomotives with control of the motors by means of thyristors for AC drive and by means of choppers with DC drive cause the appearance in the current providing rails of parasite frequencies with high levels which are, either harmonics of the basic frequency of the drive current for AC, or harmonics of the control frequency of the choppers in the case of DC drive.
Attempts have been made to modulate the carrier frequencies of the signals for energizing the track circuits so as to, on the one hand, allow transmission on the track towards the machine and, on the other hand, provide efficient protection against the harmonics of the drive current.
Such a process for coding track circuits is described for example in French Pat. No. 70.00325 filed on Jan. 7, 1970 in the name of the Applicant company and published under the U.S. Pat. No. 2,076,219. According to this patent, the modulation frequencies are very low so that the modulation index is sufficiently high and, in a particular application, the energizing frequencies for the track circuits are of the order of a kilohertz, whereas the modulation frequencies are of the order of about ten hertz.
The different means for modulating, at transmission, the energizing frequencies of the different track circuits must naturally present a safety problem. In fact, the transformation of a modulating signal into another signal may be contrary to safety. It is therefore important to have a device which guarantees that the modulated carrier frequency is indeed modulating by the proper frequency. In present installations, it is possible to disturb the modulating signal, either by superposition of a parasite signal with the modulating signal, or by erratic cuts or short-circuits, due for example to the vibration of components or connections between the modulation source and the modulated member.